Electrical supplies are used in a wide variety of applications to supply a controlled reference voltage or current. One kind of widely used voltage reference supply is a band-gap circuit, which has typically been used to provide a low reference voltage with stability in the presence of temperature variations and noise or transients. In one form of band-gap circuit, described in the article “A simple Three-Terminal IC Bandgap Reference” in IEEE Journal of Solid-State Circuits, vol. SC9, no 6, December 1974, two groups of junction-isolated bipolar transistors run at different emitter current densities. The difference in emitter current densities produces a related difference between the base-emitter voltages of the two groups. This voltage difference is added to the base-emitter voltage of the transistor with higher emitter current density with a suitable ratio defined by a voltage divider. The temperature coefficient of the base-emitter voltage is negative and tends to compensate the positive temperature coefficient of the voltage difference.
Other examples of electrical supplies include current mirror circuits, in which two groups of transistors, such as Field-Effect Transistors (FET's), are cross-coupled, so that an output branch reproduces a desired current in a reference branch, whatever the output load it supplies. By using FETs of a current conducting area several times bigger than in the reference branch, for example an array of transistors in the output branch, the reference current can be multiplied by a similar factor. Current mirror circuits are used in a wide variety of applications, such as current sources or a voltage regulators.
Such electrical supplies are themselves supplied with power. It is important to ensure that the electrical supply starts operation reliably when the source of power is first connected or switched on. Differences of component characteristics due to manufacturing tolerances may mean that, even in a design whose ideal characteristics are expected to ensure reliable start-up, a proportion of the actual production fails to start, either at all, or at least within an acceptable time.
It is known to provide start-up circuit elements that themselves start reliably and provide a start signal, triggering start-up of the electrical supply. However it is important to minimise the additional components used in the start-up circuit elements. Also some start-up circuit elements start reliably only if the build-up of voltage from the source of power is sufficiently rapid. Moreover, in certain applications it is also necessary to avoid the start-up circuit elements introducing additional current consumption, at least after the start-up phase is completed, if their utility occurs only when power is first applied and additional power consumption in the start-up circuit would be wasteful during normal operation of the electrical supply, especially in the quiescent state of the electrical supply.
U.S. Pat. No. 5,867,013 describes a start-up circuit but needs an additional low voltage source to supply the start-up circuit. European patent specification 1 102 400 describes a start-up circuit for a band-gap circuit but has a residual static current when the output band-gap circuit has started. U.S. Pat. No. 2004/0 124 823 describes a start-up circuit but needs a regulated low voltage power supply for the start-up circuit. U.S. Pat. No. 5,742,155 describes a start-up circuit but needs an additional logic signal to command the start-up circuit.
The present invention addresses some or all of these issues.